The present invention relates to an echo canceling method or apparatus that can simultaneously cancel echoes occurring in plural transmission lines.
Regarding the techniques of canceling echoes leaking from the transmitter side to the receiver side on the 4-line side of a 2-4 wire hybrid transformer, an echo canceler disclosed in xe2x80x9cAdaptive Signal Processingxe2x80x9d, 1985. Practice-Hall Inc., USA (reference 1xe2x80x94see Appendix 1) is known.
An echo canceller uses an adaptive filter with tap coefficients equal to or exceeding the impulse response length of the echo path. The echo canceller generates a pseudo echo (echo replica) corresponding to the transmitted signal and then suppresses the echo leaking from the transmitter side to the receiver side in the 2-4 wire hybrid transformer. Each tap coefficient of the adaptive filter can be adapted by taking the correlation between the transmitted signal and an error obtained by subtracting the echo replica from a mixed signal containing the echo and the received signal.
As a typical coefficient adaptation algorithm for the adaptive filters, the normalized LMS (NLMS) disclosed in xe2x80x9cAdaptive Filtersxe2x80x9d, 1985, Kulwer Academic Publications, USA (reference 2xe2x80x94see Appendix 2) is known.
In actual communication lines, plural subscriber lines are multiplexed to further improve the communication capacity for efficiency. In a such case, echo cancellers that remove echoes in the 2-4 wire hybrid transformer are provided corresponding to the number of lines to be multiplexed.
FIG. 31 shows a configuration of a prior art of multiplexed echo cancellers where the number of multiplex is three. In the first channel, a transmission signal is input to the transmission signal input terminal 1 and then is transmitted to the transmission line via the transmission signal output terminal 2. The transmission signal is further transmitted to the 2-line side of the 2-4 wire hybrid transformer 3. However, the mismatch in impedance causes a part of the transmission signal to leak as an echo into the receiving side. The echo is supplied to the subtracter 5 via the received signal input terminal 4. The adaptive filter 86 receives the input signal 700 input to the input terminal 1 and then performs the convolution of the input signal with a corrected coefficient value of the adaptive filter 86 based on the error signal 702 which is the output of the subtracter 5, thus, creating an echo replica 701. The subtracter 5 subtracts the echo replica 701 given by the adaptive filter 86 from the echo leaking into the receiver side and then transmits the subtracted result to the received signal output terminal 6. The subtracted result is further fed back to the adaptive filter 86 as the error signal 702 for coefficient updating. FIG. 32 is a block circuit diagram illustrating a configuration of the adaptive filter 86. The adaptive filter 86 has (Nxe2x88x921) delay elements including the delay elements 20l to 20Nxe2x88x921 each of which delays the transmission signal 700. The number of taps, including the tap with a delay of zero, is N. The adaptive filter 86 further has N coefficient generators 310lto 310N to generate tap coefficients thereof. N delayed samples, or outputs of the delay elements, are supplied to the corresponding coefficient generators 310l to 310N and the multipliers 40l to 40N.
The multipliers 40l to 40N respectively multiply tap coefficients output from the coefficient generators 310l to 310N by delayed samples from corresponding delay elements and then outputs the results to the adder 8. The adder 8 adds all the results from the multipliers 40l to 40N and then outputs the sum as the echo replica 701. The memory 105 supplies a step size used for coefficient adaptation to with the coefficient generators 310lto 310N.
The coefficient generator 310i (i=1, 2, . . . , N) has the configuration shown in FIG. 33. The multiplier 31 multiplies the error 702 by a step size. The multiplier 32 multiplies the resultant product by a delayed signal from each of the delay elements 20l to 20Nxe2x88x921. The output of the multiplier 32 represents a coefficient correction amount. The adder 33 adds the output of the multiplier 32 and a coefficient value stored in the memory 34 and then feeds back the resultant sum to the memory 34. The value delayed by the memory 34 becomes the coefficient value after updating.
The configuration of echo cancellers in the second and third channels shown in FIG. 31 is similar to that in the first channel. The operation of each element is similar to that in the first channel. Hence, the duplicate description will be omitted here.
The conventional echo canceller for multiplexed lines requires computations which increase in proportion to the number of multiplexed lines. This results from the fact that the characteristics of signals input to the multiplexed line are not considered.
The present invention is made to solve the above-mentioned problems.
The objective of the invention is to provide an echo canceling method suitable for multiplexed lines, which requires a small amount of computations.
Furthermore, the objective of the present invention is to provide an echo canceling apparatus suitable for multiplexed lines, which requires a small amount of computations.
The echo canceling method or apparatus for multiplexed lines of the present invention is characterized in that the convergence degrees of adaptive filters in plural channels are mutually compared and the coefficient updating of adaptive filters whose convergence is leading is suppressed.
Specifically, the echo canceling apparatus has a controller-that evaluates a set of information regarding time-varying step sizes received from adaptive filters respectively connected to transmission lines,. thus, respectively supplying coefficient update suppression signals to the adaptive filters.
Moreover, the echo canceling apparatus has a controller that receives a set of information regarding tap coefficient positions, in the area of the tapped delay line where tap coefficients are most concentrated, from adaptive filters respectively connected to transmission lines, and evaluates variations of the information, thus, supplying coefficient update suppression signals to the respective adaptive filters.